The long term goal of this project is to understand the biochemical processes of cell migration along a chemical gradient. The leucocyte has proven to be a suitable system for this purpose, and studies are proceeding in two directions: on the structural requirements of attractants for recognition by the cells, and on the subsequent molecular events that result in chemotaxis upon the interaction between attractant and cells. It has been found that small formylmethionyl peptides, possibly related to bacterial factors, are potent attractants for leucocytes. These compounds have been used to probe the migratory process in leucocytes and evidence has been adduced to implicate the role of hydrolytic enzymes not only in chemotaxis, but also in the release of histamine and lysosomal enzymes as well. In addition, further information has been obtained on structure-activity relationships of attractants. A specific chemotactic receptor on the neutrophil has also been demonstrated, and a role for methylation has become evident. Therapeutic applications of these peptides are being explored.